Flexible permanent magnetic materials are often supplied in the form of sheets or rolls and have been commercially available for many years. These materials are typically prepared by mixing a powdered ferrite material with a suitable polymeric or plastic binder into a uniform mixture. The polymeric materials are often elastomers, and the process is therefore typically accomplished through the use of sheet extrusion or calendering. The mixture is converted into strip or sheet form, providing a permanent stable product that is usually somewhat flexible, and that can readily be handled and made into elements of any desired shape by cutting and/or stamping.
The magnetic material is permanently magnetized so that the resulting elements can act individually as permanent magnets, the magnetic field being of sufficient strength that they will adhere to a magnetically attracted material, such as an iron or steel sheet. Many magnetic materials and the resultant sheet materials are typically inherently dark in color and it is therefore usual to attach these magnets to a printable substrate such as paper or plastic by gluing. A decorative pattern and/or other information may be printed on that paper or plastic. A popular application of such materials is thin, flat magnets having on their outer surface a decorative pattern and/or promotional information, including advertisements in direct mailings, newspaper inserts, and so forth, box toppers, coupons, business cards, calendars, greeting cards, postcards, and so forth.
A minimal force, Fmin, of attractive interaction between a magnetizable plate and a magnetic member is defined by the following expression:Fmin>Fg·Kfr  (equation 1)
where Fg is a force applied to the magnetic member by the hanged object due to gravity; Kfr is a coefficient of friction between the magnetizable plate and the magnetic member.
The force of magnetic attraction between two substantially flat bodies where one of them is a flat permanent magnet and another is a plate made of a magnetizable material depends on the following parameters: (a) magnetic moment per unit volume of the magnet; (b) magnet volume; (c) magnetization value of the magnetizable material; and (d) contact area between the magnet and magnetizable member.
The force Fmin is proportional to each abovementioned parameters.
It is believed that the technology relevant to the present invention is disclosed in: U.S. Pat. No. 3,987,567 (Fritts, 1976 Oct. 16): US 2003/106,913 (Kohno, 2003 Jun. 12); US 2008/087,785 (Roche, 2008 Apr. 7); and JP 2005/218,487 (Ogushi, 2010 Aug. 18), all of which are incorporated herein by reference.